Several systems for determining the angular location of a single external light source are disclosed in prior patents. A system using a plurality of detectors positioned behind an adjustable mask is disclosed in Reymond et al., U.S. Pat. No. 4,314,761 issued Feb. 9, 1982. The system so disclosed however, is unable to distinguish between multiple sources. Although it is used in conjunction with a number of external sources, such sources are illuminated one at a time and to be distinguished, the sequential illumination of these sources must be precisely phased with the detecting circuitry. Because of its inability to identify multiple simultaneous beacons, this system, when confronted with multiple sources, will register spurious position information or `ghosts`, the total number of registered images being equal to the square of the number of sources in the field of view.
U.S. Pat. No. 4,112,294, Pressiat, et. al., discloses a similar system comprising a four quadrant mask and a single detector. Again, as is clear from the disclosure, such a system is unable to distinguish between multiple sources. When confronted with resolvable multiple sources, this latter system will ignore one. Which source is ignored, in general, will be unpredictable.